We have previously described a cylindrical ThermoPhotoVoltaic (TPV) generator in which combustion is used to heat a cylindrical tube to a temperature in the range of 1200 C. to 1500 C. This glowing tube then emits infrared radiant energy. An array of low bandgap photovoltaic (PV) cells surrounds this glowing emitter, receives the infrared radiant energy, and converts it to electricity. Our previously described cylindrical TPV generator was also equipped with a recuperator used to recycle heat from the combustion exhaust stream back into the combustion air for a higher efficiency TPV generator. We have noted that these cylindrical TPV generators can potentially be used to generate heat and power for the home with 90% fuel utilization efficiency.
In a separate more recent patent, we have described an AntiReflection coated Refractory Metal (AR/RM) IR emitter designed to emit infrared energy primarily in a wavelength band that the PV cells can convert. However, many of these AR/RM emitters cannot be operated in air because of oxidation of the refractory metal. In our specific TPV systems, we use GaSb PV cells that respond to IR wavelengths out to 1.8 microns and AR coated tungsten foil as the key emitter and receiver components. However in our previous patents, we have noted that various other material combinations are possible as long as they operate in this IR wavelength range.
There is a need to design a cylindrical TPV generator with a hermetic seal that allows the AR/RM emitter to operate in a non-oxidizing inert gas atmosphere. This hermetic seal design must minimize heat transfer from the very hot emitter support tube to the hermetic seal. Furthermore, for the home Combined Heat and Power (CHP) application, there is a need for a low NOx burner/recuperator combination. The recuperator design needs to be compact, inexpensive to fabricate, and integrated with the hermetic seal and low NOx requirements. Finally, the generator design needs to produce electricity with a conversion efficiency as high as possible.
Our cylindrical TPV generator uses low bandgap PV cells mounted on circuits in a polygonal array around an IR emitter. The IR emitter is located on the outside surface of a radiant tube burner coaxial with the PV array. The combustion gases are completely contained within the radiant tube burner. The PV array is mounted inside a leak-tight envelope cooled on its outer surface by either water or air flow. Flanges on either end of this PV array container allow for hermetic seals. The flange on one end of this PV container seals to a flange on the end of the emitter support tube. This seal allows the space between the emitter and the PV array to be back-filled with an inert gas. However in order to avoid overheating this seal, the emitter support tube is elongated extending into the recuperator section and then folded back exiting the recuperator with a slightly larger coaxial tube connecting to a flange sealing to the Photovoltaic Converter Array (PCA). This folded back coaxial emitter support tube provides a long path length limiting thermal conduction along its cylindrical wall from the very hot emitter section to the cooled seal flange.
The folded back emitter support tube blends nicely with a two stage folded back recuperator assembly consisting of two sets of finned disks. A stack of smaller finned disks is located inside the emitter support tube and a second stack of larger finned disks is located outside the fold back section. The dual disk stack design has several advantages. First, it is very compact being much shorter in length than a single disk stack. Second, it is more efficient than a single disk stack because the hottest section is inside cooler sections. In a single disk stack, the outer section is the hottest. The third advantage is somewhat subtle. This design allows for a low NOx burner/recuperator assembly.
It is desirable for a home TPV generator to operate with low NOx. However, high NOx can be a problem for a high temperature burner. A solution to this NOx problem is to burn the fuel at high temperature with no excess oxygen so that little NOx is generated. However, this fuel rich burn leaves CO and hydrocarbons. These can be eliminated in a low temperature after-burn with a catalyst. In our improved cylindrical TPV generator, we provide for a low temperature catalytic after-burn by providing a perforated turnaround plate coupling between the inner disk stack and the outer disk stack. This perforated turnaround plate provides a small amount of combustion air for the after-burn. A catalyst coating can be provided on the hotter surface of the outer finned disks. The after-burn occurs in the outer finned disk stack.
Additional features are incorporated in our cylindrical TPV generator to provide for high conversion efficiency. Mirrors are located at the ends of the PV array to confine the IR energy between the emitter and the array. Also, the inner burner tube within the emitter support tube can be tapered in order to optimize the emitter temperature profile along the length of the emitter. Both of these provisions provide for uniform illumination of the cells so that all of the cells in a series string generate approximately the same current.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.